Immunoglobulin synthesis in individual lymphocytes is regulated in such a way that only one of two possible light chain genes is expressed by the cell. Thus, in heterozygous cells only one of the two possible light chain alleles is expressed. This phenomenon, allelic exclusion, is known to occur only in lymphocytes; its mechanics are unknown. We have observed allelic exclusion in cell-free peptide-synthesizing systems derived from genetically marked rabbit lymphocytes. We have data showing that allotype synthesis is determined by the genetic characteristics (homozygous b4 or b5 allotype, or heterozygous b4b5 allotype) of the cells used to prepare the non-particulate component of the reaction mix needed for cell-free peptide synthesis. We intend to isolate and identify the factor or factors that exclude the production of one allotype peptide while allowing or mediating the production of the other. These factors will be isolated from purified, stimulated, B cells in culture by standard prparative sucrose gradient centrifugation, DEAE-cellulose, carboxymethyl-cellulose chromatogrphy, and acrylamide gel electrophoresis procedures. The effects of the factor on cell-free allotype synthesis and on cellular allotype synthesis will be followed by a radio-precipitation method measuring interference of interaction of quantitated I125-IgG peptides with cellulose bound allotype-specific antibody. BIBLIOGRAPHIC REFERENCES: Teodorescu, M., E. P. Mayer, H. Reiter, and S. Dray, 1976. Rabbit lymphocyte subpopulations. 1. Separation of Ig plus and Ig minus cells and their interactions in cultures stimulated by mitogens. Cell. Immunol. 22: 66-75. Meyer, E. P., M. Teodorescu, H. Reiter, and S. Dray, 1976. Rabbit lymphocyte subpopulations. II. Separation and characterization of two Ig plus cell subpopulations. Cell. Immunol. 22: 76-82.